1. Field of the Invention
The present invention relates to a thermal transfer sheet, which is suitable as a donor element for image-wise preparing a lithographic printing form cylinder. More particularly, the present invention is a thermal transfer sheet for laser induced transfer of an image to an offset-printing form cylinder having a substrate layer and a doner layer on the substrate layer.
The invention also relates to a method of making the thermal transfer sheet as well as to intermediate products therefor.
2. Description of the Related Art
A printing method is known wherein a printing form cylinder is provided with an image dot by dot using resin particles. This printing form cylinder is used in an offset-printing operation, during which it is coated with ink. The ink is then transferred to a rubber roll and is thereafter transferred from the rubber roll to the substrate, normally paper. In order to alter the printing motif in short time intervals, particularly if small issues are to be printed, it is desirable to carry out the operation using only one piece of machinery controlled by a computer. The printing device as described in-EP-B-0 698 488, to which U.S. Pat. No. 5,601,222 corresponds, meets these requirements.
The printing form cylinder used in the above-mentioned printing device is coated with a polymer from a thermal transfer sheet dot by dot with an image. In order to obtain a printing plate being suitable for offset-printing, i.e. having a clear separation between hydrophilic (i.e. parts of the printing form cylinder not coated by the polymer) and hydrophobic (i.e. parts of the printing form cylinder coated by the polymer, which later during the printing operation absorb the printing ink) portions, certain physical and chemical parameters of the thermal transfer sheet, especially a thermal transfer ribbon, have to be optimized. It has been shown that common thermal transfer sheets do not or only partly fulfill these requirements.
The object of the present invention is to provide a thermal transfer sheet or ribbon suitable for preparing a printing form cylinder by laser-induced transfer of a polymer from a donor layer of the thermal transfer sheet or ribbon to the printing form cylinder. The thermal transfer sheet or ribbon must fulfill the following requirements: The support or substrate must fulfill mechanical requirements during transfer, optical requirements of the light transmission when irradiated with a laser beam and thermal requirements of heating of the coated layer. The coating on the metal should strongly adhere to the metal body and should have a uniform printing quality over a sufficient life-time. Finally removal of the coated polymer layer from the printing plate should be possible under mild and environmental-friendly conditions and in a short period of time to allow a new printing operation to be started.
Briefly stated, the present invention is a thermal transfer sheet having a substrate layer and a donor layer. The substrate layer includes a first polymer having mechanical stability at temperatures greater than 150 degrees C. and light transmission of at least 70% for wavelengths of from about 700 nm to about 1600 nm. The donor layer includes a second polymer having at least one of acidic groups and optionally substituted amide groups and an additive capable of converting incident laser light energy into heat energy.